Hi, I'd like to help you with your question.
There are a variety of reasons someone might isolate themselves from family. For example, there may have been a disagreement you are not aware of or they may have personal issues going on they do not want to share or may not be able to handle contact for other reasons. Sometimes feelings get the best of someone and they need time and space to work it out. It may also be a reason that has nothing to do with family and is just something they need to address on their own.
You listed numerous things you have tried already. All the actions you listed were right on target. They would be exactly what I would suggest. Beyond these ideas, there is little else you can do. There is no way to force someone to have contact. The only thing you can do is allow her to work out whatever the issue is on her own. Let her know you are there for her whenever she is ready to have contact again.
Continue to send cards on holidays and birthdays and an occasional email. She will come around when she is ready. Hopefully, she will see that you care and be able to respond.
I hope this helped you,